Gravitational Fields Flashcards
What is meant by a force field
An area in which an object experiences a non contact force
Describe the similarities between gravitational and electric fields
- Forces both follow inverse square laws
- Use field lines to be represented
- Both have equipotential surfaces
Describe differences between gravitational and electric fields
- For gravitational force exerted is always attractive
- Where as electric field the force can be either repulsive or attractive
DO NOT SAY POSITIVE OR NEGATIVE
- Electric force acts on charge, while gravitational force acts on a mass
Newtons law of gravitation
Magnitude of force between two objects
- directly proportional to product of masses
- inversely proportional to the square of distance between them (hence inverse square law)
Definition of gravitational field strength
force per unit mass exerted by a gravitational field on an object
Equation for gravitational field strength for radial and uniform fields (two diff ones)
g =F/m
g = GM/r^2
Definition of gravitational potential (V)
Work done per unit mass when moving an object from infinity to that point
Describe why gravitational potential is always negative
- Gravitational potential at infinity is zero
- As the object moves from infinity to a point, energy is released
(As the gravitational potential energy is reduces) - Therefore gravitational potential must always be negative (since reducing from 0)
Definition of gravitational potential difference
Energy needed to move a unit mass between two points
Describe why gravitational potential difference is useful
- It can be used to find the work done when moving an object in a gravitational field between two points
Since it is energy needed = work done
What is known about equipotential surfaces
Potential on an equipotential surface is constant everywhere
- Therefore no work is done when moving along an equipotential surface
Describe the graph for gravitational potential against radius
Within the negative quadrant for the y axis
A curve tending towards the x axis (sort of a reciprocal)
Where the curve starts is the radius of the graph
The gradient is -g
Describe the graph of gravitational field strength against radius
Inverse proportional graph shape
Area under points on graph is gravitational potential difference
Explain how you can work the derivation of Kepler’s 3rd law
Centripetal force = gravitational force
Rearrange for v
Velocity is rate of change of momentum
Sub into v and solve for T
What is the total energy of a satellite
Kinetic energy + potential energy (its constant)
What is meant be escape velocity
The minimum speed that will allow an object to escape a gravitational field with no further energy input
In which the object will not fall back into the gravitational field due to attraction
At what point is escape velocity reached in terms of energy
Object reaches escape velocity when its kinetic energy has been transferred to gravitational potential energy
Derive the equation for escape velocity
KE = GPE
0.5 x m x v^2 = GMm / r
Little m’s cancel and solve for v
v = root 2GM/r
What can we infer about escape velocity for different objects from the equation
Escape velocity will be the same for all objects
It is independent of mass of the object
What is a synchronous orbit
When an orbiting body has a time period equal to that of the body it is orbiting
And in the same direction of rotation as that body
What is a geosynchronous orbit
A synchronous orbit that is directly above the equator
Which direction is the Earths rotation
West to east
What is the orbital time period of Earth
24 hours
What are the key features of geostationary orbit
- Equatorial orbit
- Moves west to east
- Period of 24 hours
